FCC process with spent catalyst recycle

ABSTRACT

Disclosed is an FCC apparatus and process in which coked catalyst is recycled to the base of the riser to contact fresh feed through a passage disposed within the riser.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Division of copending application Ser. No.11/957,952 filed Dec. 17, 2007, the contents of which are herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The field of the invention is the fluid catalytic cracking (FCC) ofheavy hydrocarbons into lighter hydrocarbons with a fluidized stream ofcatalyst particles.

DESCRIPTION OF THE PRIOR ART

Catalytic cracking is accomplished by contacting hydrocarbons in areaction zone with a catalyst composed of finely divided particulatematerial. The reaction in catalytic cracking, as opposed tohydrocracking, is carried out in the absence of added hydrogen or theconsumption of hydrogen. As the cracking reaction proceeds, substantialamounts of coke are deposited on the catalyst. The catalyst isregenerated at high temperatures by burning coke from the catalyst in aregeneration zone. Coke-containing catalyst, referred to herein as“coked catalyst”, is continually transported from the reaction zone tothe regeneration zone to be regenerated and replaced by essentiallycoke-free regenerated catalyst from the regeneration zone. Fluidizationof the catalyst particles by various gaseous streams allows thetransport of catalyst between the reaction zone and regeneration zone.

Patents disclose processes that use catalyst recycle withoutregeneration. U.S. Pat. No. 3,888,762 discloses sending strippedcatalyst and regenerated catalyst to the base of the riser withoutmixing. U.S. Pat. No. 4,853,105 discloses an FCC process wherebystripped, coked catalyst is recycled to the riser just less than mid-wayup the riser. U.S. Pat. No. 5,858,207 discloses an FCC process whereinregenerated catalyst and stripped coked catalyst are subjected tosecondary stripping before being returned to the riser to contact feed.U.S. Pat. No. 5,372,704 discloses an FCC process wherein coked catalystfrom a first FCC unit is charged to a riser of a second naphtha crackingunit and then recycled back to the riser of the first FCC unit. U.S.Pat. No. 5,597,537 discloses recycling stripped catalyst to a vessel inwhich it is mixed with regenerated catalyst and contacted with feed in ariser.

In typical cases, coked catalyst is recycled to the base of the riser byuse of a recirculation conduit or standpipe. The standpipe may utilize aslide valve to control catalyst flow rate, an expansion joint to accountfor thermal expansion and hangers and guides to absorb horizontal andvertical loads, respectively.

It would be advantageous to provide for coked catalyst recycle withoutuse of a standpipe.

SUMMARY OF THE INVENTION

I have discovered that a recycle conduit disposed within the shell of anFCC riser negates the need for an FCC recycle standpipe. The inventionalso negates the need for the equipment associated with the recyclestandpipe.

Additional objects, embodiments, and details of this invention willbecome apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic elevational view showing a FCC unit in accordancewith the present invention.

FIG. 2 is a sectional view taken from segment 2-2.

FIG. 3 is an alternative sectional view of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

This invention is more fully explained in the context of an FCC processthat recycles a portion of the coked catalyst without regeneration tothe reaction zone. FIG. 1 shows a typical schematic arrangement of a FCCunit arranged in accordance with the present invention. The descriptionof this invention in the context of the specific process arrangementshown is not meant to limit it to the details disclosed therein.

The FCC arrangement shown in FIG. 1 consists of a reactor vessel 10, aregenerator vessel 12, and a reactor riser 16 that provides a pneumaticconveyance zone in which conversion takes place. The arrangementcirculates catalyst and contacts feed in the manner hereinafterdescribed.

The catalyst comprises any of the well-known catalysts that are used inthe art of fluidized catalytic cracking, such as an active amorphousclay-type catalyst and/or a high activity, crystalline molecular sieve.Molecular sieve catalysts are preferred over amorphous catalysts becauseof their much-improved selectivity to desired products. Zeolites are themost commonly used molecular sieves in FCC processes. Preferably, thefirst catalyst comprises a large pore zeolite, such as an Y-typezeolite, an active alumina material, a binder material, comprisingeither silica or alumina and an inert filler such as kaolin. A catalystadditive may comprise a medium or smaller pore zeolite catalystexemplified by ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38, ZSM-48,and other similar materials. U.S. Pat. No. 3,702,886 describes ZSM-5.

FCC feedstocks, suitable for processing by this invention, includeconventional FCC feeds and higher boiling or residual feeds. The mostcommon of the conventional feeds is a vacuum gas oil which is typicallya hydrocarbon material having a boiling range of from 343° to 552° C.(650° to 1025° F.) and is prepared by vacuum fractionation ofatmospheric residue. Heavy or residual feeds, i.e., boiling above 499°C. (930° F.), are also suitable. The FCC process of the presentinvention is suited best for feed stocks that are heavier than naphtharange hydrocarbons boiling above about 177° C. (350° F.).

Looking then at FIG. 1, the riser reactor 16 provides a conversion zonefor cracking of the feed hydrocarbons. The vertical riser 16 may have aninner riser 18 disposed within an outer shell or wall 20. The risertypically operates with dilute phase conditions above the point of feedinjection wherein the density is usually less than 320 kg/m³ (20 lb/ft³)and, more typically, less than 160 kg/m³ (10 lb/ft³). Feed is introducedinto the inner riser 18 by one or more nozzles or distributors 22between an entrance 24 to the riser and substantially upstream from anoutlet 30. Volumetric expansion resulting from the rapid vaporization ofthe feed as it enters the riser further decreases the density of thecatalyst within the riser to typically less than 160 kg/m³ (10 lb/ft³).Before contacting the catalyst, the feed will ordinarily have atemperature in a range of from 149° to 316° C. (300° to 600° F.).Additional amounts of feed may be added downstream of the initial feedpoint.

The catalyst and reacted feed vapors are then discharged from the top ofriser 16, specifically, the inner riser 18, through the outlet 30 andseparated into a cracked product vapor stream including cracked productsand a collection of catalyst particles covered with substantialquantities of coke and generally referred to as “coked catalyst.” In aneffort to minimize the contact time of the products with the catalystwhich may promote further conversion of desired products to undesirableother products, any arrangement of separators may be used to removecoked catalyst from the product stream quickly. In particular, a swirlarm arrangement 26, provided at the end of riser 16 can further enhanceinitial catalyst and cracked hydrocarbon separation by imparting atangential velocity to the exiting catalyst and cracked product vaporstream mixture. The swirl arm arrangement is located in an upper portionof a disengaging chamber 32, and a stripping zone 34 is situated in thelower portion of the chamber 32. The disengaging chamber is indownstream communication with the outlet 30. Catalyst separated by theswirl arm arrangement 26 drops down into the stripping zone 34. Thecracked product vapor stream comprising cracked hydrocarbons and somecoked catalyst exit the chamber 32 via conduit 36 in upstreamcommunication with cyclones 38. The cyclones 38 in downstreamcommunication with the outlet 30 via conduit 36 remove remainingcatalyst particles from the product vapor stream to reduce particleconcentrations to very low levels. The product vapor stream then exitsthe top of reactor vessel 10 through outlet 40 via plenum chamber 35.Catalyst separated by cyclones 38 return to the reactor vessel 10through dipleg conduits 39 into dense bed 41 where it will enter thestripping zone 34 in the disengaging chamber 32 through openings 42.

The stripping zone 34 removes hydrocarbons entrained with the cokedcatalyst and hydrocarbons adsorbed on the surface of the catalyst bycounter-current contact with an inert gas such as steam. Baffles 43 and44 may facilitate contact of the steam with the coked catalyst. Otherstripping internals may be suitable. Inert gas enters stripping zone 34through line 46. A bed 47 of stripped catalyst may accumulate at thebase of the stripping zone 34 which can be designated a distributionzone 51 in the disengaging chamber 32.

The present invention recycles a first portion of the coked catalyst tothe riser 16 without undergoing regeneration via a passage 50 disposedwithin a shell or outer wall 20 of the riser reactor 16. A secondportion of the coked catalyst is regenerated in the regenerator 12before it is delivered to the riser 16. The first and second portions ofthe catalyst may be blended in a blending vessel (not shown) or in ablending zone 52 in the base of the riser reactor before introductioninto the inner riser 18 through entrance 24. The length of the innerriser 18 and the passage 50 are disposed within the outer wall 20 of theriser 16. The inner riser 18 and the passage 50 are isolated from eachother. The riser reactor 16 may have an inner wall 54 that with theouter wall 20 defines passage 50. In an embodiment, the passage 50 maybe an annulus. One or more feed distributors 22 may pass through thepassage 50, in which case the feed distributors may have to beappropriately lined with a shield and/or refractory. Alternatively, thepassage 50 may be a pipe. The inner riser 18 is in downstreamcommunication with the feed distributor 22. However, because the feeddistributor 22 may be in downstream communication with the entrance 24,a portion of the inner riser 18 may be upstream of the feed distributor.In another embodiment, one or more inner walls may each form a singletunnel that provides the passage 50. An inner surface of the inner wallmay define the inner riser 18. The first portion of coked catalyst maypass from the stripping zone 34 through an entrance 62, into the passage50, out an outlet 70 and into the mixing zone 52. The stripping zone 34is in upstream communication with the distribution zone 51 which is inupstream communication with an entrance 62 to the passage 50. Hence, theentrance 62 to the passage 50 is in downstream communication with theoutlet 30. It is contemplated that coked catalyst be transported throughpassage 50 without undergoing stripping in stripping zone 34. One weir60 or more extending from the outer wall 20 may define with the innerwall 54 the entrance 62 or more to the passage 50. It is envisaged thatweir 60 can pivot or be fixed with a predetermined spacing definingentrance 62 and set at an appropriate elevation to generate anappropriate pressure drop to allow flow or a particular flow rate intothe passage 50 when the catalyst bed 47 attains a predetermined level.Levees, hoppers or other suitable devices may be used instead of a weir60 system.

The second portion of the coked, stripped catalyst is transported to theregeneration zone through coked catalyst conduit 58 in downstreamcommunication with the distribution zone 51 at a rate regulated bycontrol valve 59 for the removal of coke from the catalyst. The controlvalve 59 may also be used to control the depth of the catalyst bed 47 inthe stripping zone 34 which may control the flow rate of coked catalystinto passage 50.

On the regeneration side of the process, coked catalyst transferred toregenerator 12 via conduit 58 undergoes the typical combustion of cokefrom the surface of the catalyst particles by contact with anoxygen-containing gas. The regenerator vessel 12 may be a combustor-typeof regenerator, which may use hybrid turbulent bed-fast fluidizedconditions in a high-efficiency regenerator vessel 12 for completelyregenerating coked catalyst. However, other regenerator vessels andother flow conditions may be suitable for the present invention. Thereactor conduit 58 feeds coked catalyst to a first or lower chamber 68.The coked catalyst from the reactor vessel 10 usually contains carbon inan amount of from 0.2 to 2 wt-%, which is present in the form of coke.An oxygen-containing combustion gas, typically air, enters the firstchamber 68 of the regenerator 12 through a conduit 64 and is distributedby a distributor 66. Openings in the distributor 66 emit combustion gas.As the combustion gas contacts coked catalyst, it typically lifts thecatalyst under fast fluidized flow conditions. The lifted catalyst mayhave a catalyst density of from 48 to 320 kg/m³ (3 to 20 lb/ft³) and thecombustion gas may have a superficial gas velocity of 1.1 to 2.2 m/s(3.5 to 7 ft/s) in the first chamber 68. The oxygen in the combustiongas contacts the coked catalyst and combusts carbonaceous deposits fromthe catalyst to at least partially regenerate the catalyst and generateflue gas. Hot regenerated catalyst from a dense catalyst bed 69 in anupper or second chamber 100 may be recirculated into the first chamber68 via an external recycle standpipe 67 regulated by a control valve toraise the overall temperature of the catalyst and gas mixture in thefirst chamber 68.

The mixture of catalyst and flue gas in the first chamber 68 ascendthrough a frustoconical transition section 72 to the transport, risersection 74 of the first chamber 68. The mixture of catalyst and gasaccelerates through the reduced cross-sectional area of the risersection 74. Hence, the superficial gas velocity will usually exceedabout 2.2 m/s (7 ft/s). The riser section 60 will have a lower catalystdensity of less than about 80 kg/m³ (5 lb/ft³).

The regenerator vessel 12 also includes an upper or second chamber 100.The mixture of catalyst particles and flue gas is discharged throughdisengaging arms 82 from an upper portion of the riser section 74 intothe separation chamber 100. Substantially completely regeneratedcatalyst may exit the top of the transport, riser section 74, butarrangements in which partially regenerated catalyst exits from thefirst chamber 68 are also contemplated. Discharge is effected throughdisengaging arms 82 that separate a majority of the regenerated catalystfrom the flue gas. The catalyst and gas exit through downwardly directedopenings in disengaging arms 82. The sudden loss of momentum anddownward flow reversal cause most of the heavier catalyst to fall to thedense catalyst bed 69 and the lighter flue gas and a minor portion ofthe catalyst still entrained therein to ascend upwardly in the secondchamber 100. A fluidizing conduit 106 delivers fluidizing gas, typicallyair, to the dense catalyst bed 69 through a fluidizing distributor 108.

The combined flue gas and fluidizing gas and entrained particles ofcatalyst enter one or more cyclone separators 98, 99, which separatescatalyst fines from the gas. Flue gas, relatively free of catalyst iswithdrawn from the regenerator vessel 12 through an exit conduit 110while recovered catalyst is returned to the dense catalyst bed 69through respective diplegs 112, 113.

Regenerated catalyst conduit 78 passes regenerated catalyst fromregenerator 12 into the blending zone 52 at a rate regulated by controlvalve 76 where it is blended with recycled catalyst exiting outlets 70after descending through passage 50. The outlets 70 and conduit 78 arein upstream communication with the blending zone 52 and entrance 24 tothe inner riser 18 is in downstream communication with the blending zone52 and the regenerated catalyst conduit 78. Fluidizing gas passed intothe blending zone 52 by distributor 14 contacts the catalyst andmaintains the catalyst in a fluidized state to blend the recycled andregenerated catalyst and lift it into the entrance 24 of the inner riser18.

The regenerated catalyst which is relatively hot is cooled by theunregenerated, coked catalyst which is relatively cool to reduce thetemperature of the regenerated catalyst by 28° to 83° C. (50° to 150°F.) depending upon the regenerator temperature and the coked catalystrecycle rate. The amount of blended catalyst that contacts the feed willvary depending on the temperature of the regenerated catalyst and theratio of recycled to regenerated catalyst comprising the catalyst blend.The term “blended catalyst” refers to the total amount of solids thatcontact the feed and include both the regenerated catalyst fromregenerator 12 and the recycled catalyst portion from the passage 50.Generally, the blended catalyst to feed will be in a ratio of from 5 to50. Ordinarily, the ratio of recycled catalyst to regenerated catalystentering the blending zone will be in a broad range of from 0.1 to 5.0and more typically in a range of from 0.3 to 3.0. The amount of coke onthe recycled catalyst portion returning to the blending zone 52 willvary depending on the number of times the catalyst particle has recycledthrough the riser. Since the separation of the catalyst particles out ofthe riser is random, the coke content of the particles leaving the riserwill be normally distributed, varying between the coke content of aparticle going through the riser only once and the coke content of aparticle that has gone through the riser many times. Nevertheless, thecoked catalyst portion entering the regeneration zone as well as therecycled catalyst portion could range from an average coke concentrationof between 0.3 to 1.1 wt-%. The preferred range of average cokeconcentration is 0.5 to 1.0 wt-%. Moreover, the blended catalystcomposition will contain at least 0.1 wt-% coke before contacting thefeed.

Regenerated catalyst from regenerator standpipe 78 will usually have atemperature in a range from 677° to 760° C. (1250° to 1400° F.) and,more typically, in a range of from 699° to 760° C. (1290° to 1400° F.).The temperature of the recycled catalyst portion will usually be in arange of from 510° to 621° C. (950° to 1150° F.). The relativeproportions of the recycled and regenerated catalyst will determine thetemperature of the blended catalyst mixture that enters the riser. Theblended catalyst mixture will usually range from about 593° to 704° C.(1100° to 1300° F.) and, more preferably at about 649° C. (1200° F.).

Inner surface of outer wall 20 and inner wall 54 and outer surface ofinner 54 should be lined with refractory as well as weirs 60 andportions of distributors 22 that may be impacted by catalyst flow.

FIG. 2 is a sectional view of the riser 16 and recycle catalyst passage50 of FIG. 1. The recycle passage 50 is concentric with the inner riser18. An inner surface of the outer wall 20 of the riser and the outersurface of the inner wall 54 define passage 50 which provides fordownward recycle of coked catalyst. The inner riser 18 is defined byinner surface of inner wall 54. Downwardly located equipment such asfeed distributors 22 are not shown. Appropriate equipment will benecessary to support the inner riser 18 within the riser 16.

FIG. 3 is an alternative sectional view of FIG. 2 in which referencenumerals corresponding to elements with different configurationsdesignated with a prime symbol (“′”). The riser 16′ contains two recyclecatalyst passages 50′. More or less are contemplated with alternativeshapes and configurations. The recycle passages 50′ are spaced apart onopposed sides of the riser 16′. A portion of an inner surface of theouter wall 20 of the riser 16′ and the outer surface of two inner walls54′ respectively define passages 50′ which provide for downward recycleof coked catalyst. The inner riser 18′ is defined by portions of innersurfaces of the outer wall 20 of the riser 16′ and by inner surfaces ofinner walls 54′. Other alternatives may also be suitable.

1. A process for fluidized catalytic cracking of a hydrocarbon feedstream to products, the process comprising: blending recycled cokedcatalyst and regenerated catalyst to provide a blended catalyst;contacting said hydrocarbon feed stream with said blended catalystwithin an outer wall of a riser to produce said products and cokedcatalyst; regenerating a first portion of said coked catalyst to providesaid regenerated catalyst; and recycling a second portion of said cokedcatalyst through a passage disposed within said outer wall of saidriser, wherein an inlet to said passage is disposed in a disengagingchamber, the disengaging chamber being in downstream communication withan outlet of said riser.
 2. The process of claim 1 further comprisingcontacting said coked catalyst and regenerated catalyst in an innerriser isolated from said passage.